Radio wave absorbers have been used in anechoic chambers, facilities to evaluate radio noise from various electronic devices and communication devices and to check if radio waves cause a malfunction, and recently used to reduce radio wave hindrance, e.g., radio wave interference, in wireless communication systems such as an electronic toll collection (ETC) system, a wireless LAN system, and a radio-frequency identification (RFID) system.
Such radio wave absorbers absorb radio waves by converting radio energy into heat energy, and thus may be burned when irradiated with high-energy radio waves. Therefore, absorbers used particularly in anechoic chambers require flame resistance. Radio wave absorbers used in other applications are also required to have flame resistance to secure the safety from fire and the like.
In addition, such radio wave absorbers are required to be lightweight from the standpoint of handleability (processability), and further required to have high rigidity so that there is no breakage or deformation which may reduce their radio wave absorbency.
Furthermore, radio wave absorbers used in anechoic chambers, which are used for as long as 10 years or more, are also required not to undergo discoloration or reduction in flame resistance during the time period.
As a radio wave absorber that satisfies the requirements described above, JP 2005-311332 A discloses a radio wave absorber having a hollow three-dimensional structure obtained by fabricating a sheet material having flame resistance into a corrugated cardboard structure and assembling the corrugated cardboard structure. The sheet material constituting this radio wave absorber, however, requires large amounts of inorganic components (in particular, inorganic powder) to achieve the desired flame resistance. As a result, the sheet material has a low tensile strength, and even when the sheet material is fabricated into a corrugated cardboard structure, the corrugated cardboard structure is not provided with a rigidity sufficient to withstand the impact during conveyance, during construction, or after construction, and may be deformed or broken. To improve rigidity, it is necessary to increase the basis weight of the sheet material, i.e., increase the amount of raw materials used for the sheet material, which may result in increase in mass of an absorber and increase in cost. Furthermore, there are also processing problems: for example, when the sheet material is processed into a corrugated cardboard structure, the sheet material, because of its low tensile strength, may be broken under the pressure or tension of processing equipment, and the processing speed cannot be increased to prevent such poor processing, leading to increase in processing cost.
JP 2009-194341 A discloses electromagnetic wave suppressing paper obtained by impregnating base paper containing wood pulp and carbon fibers with a guanidine flame retardant such as guanidine phosphate. That composition in which large amounts of inorganic components are not used as a flame retardant can provide a sheet material with a tensile strength. However, flame retardants for cellulose, including guanidine flame retardants, may cause discoloration of wood pulp in base paper, or may be detached in a high-humidity environment to result in reduced flame resistance.
It could therefore be helpful to provide highly processable flame-resistant paper for radio wave absorber members which has a high tensile strength, a moderate rigidity, and a high flame resistance, which properties are preferred for radio wave absorber members, undergoes no discoloration such as yellowing after long-term use, and maintains its high flame resistance even in a high-humidity environment; and a radio wave absorbing member with high rigidity and excellent handleability including the same.